Continuous wave THz imaging of multi-walled carbon nanotubes polymer composites

In this paper, the problem of creating homogeneous composites with on carbon nanotubes is described. To control the quality of the manufactured composites, a system of terahertz visualization of material inhomogeneity using a continuous radiation source is used. An increase in the homogeneity of the composite based on multi-walled carbon nanotubes with an increase in the time of ultrasonic processing during polymerization is noted. The advantages of THz imaging in comparison with optical microscopy are shown.


Introduction
Currently, composite materials with various fillers from cermet inclusions [1] to ferromagnets [2] are actively studied. Multi-walled carbon nanotubes (MWCNTs) are promising nanostructured materials for use as a filler in composites. MWCNTs-based composites have unique mechanical properties [3], such as high strength and rigidity and a huge aspect ratio (the ratio of length to diameter) [4]. These properties are used in micro-and nanoelectronics as dielectrics, sensors [5]; in the chemical industry as heterodyne catalysts of chemical processes [6]; in the light industryas heavy-duty filaments [7], components of bitumen (asphalt) [8]. These unique properties are achieved by the homogeneous allocation of MWCNTs in the composite and depend on the manufacturing technology. Monitoring of the localization of MWCNTs in manufactured samples can allow assessing the homogeneity of the composite material at different manufacturing technologies.
The progress of visualization technologies today makes it possible to apply them in the art [9], archeology [10], medicine [11], food [12] and electronic industries [13]. Over the last decades, material defectoscopy at terahertz (THz) frequency range became popular [14]. The prospects for the use of THz radiation are primarily because the photon energy of radiation in the submillimeter frequency range is 0.1-0.001 eV (non-ionizing) [15]. The submillimeter-wave radiation has a high penetrating power through thin dielectric media [16]. THz imaging has a number of advantages over long-used methods of non-destructive control, such as optical [17] and X-ray [18] diagnostics. Among the advantages, low measurement error due to the simplicity of processing the results; high compatibility with biological objects [15]; technical implementation of THz systems provides usability and relatively low cost of components relative to more precise X-ray systems. The purpose of this research is to evaluate the heterogeneity of composites based on MWCNTs and determine its dependence on the time of ultrasonic treatment.

Experiment
Defectoscopy of MWCNTs composite samples was carried out using the THz imaging system [19]. This system recording of the intensity distribution of the transmitted radiation through the composite sample in a 2D plane. THz imaging system consist of the 2D positioning mechanism of investigated object placed in the quasi-optical path of the interferometer Mach-Zander. The functionality of the system provides the ability to identify the characteristics of the material included in the object by analyzing the frequency properties. Automation of the non-destructive visualization system allows to exclude the influence of human factor on the measuring process, as well as to reduce the need for operator's participation.

Materials
In this research, we used MWCNTs manufactured by the method of catalytic gas-phase deposition of ethylene. The nanotube diameter ranged from 4 to 21 nm, and ED-20 epoxy was used as the binding material.
The technology of production of composite materials samples included 5 stages. Firstly, the epoxy was mechanically mixed with 1 wt. % MWCNTs during 5 minutes. Then, influence for composites samples by 75 W power ultrasonic treatment with frequency of 28 kHz to enhance its homogeneity was performed. Then to obtain the polymer mixtures was dried at room temperature for 48 hours. Finally, the 4 composite samples were mechanically grinded until they were flat-parallel. Sample No. 1 was without ultrasonic treatment, while samples No. 2-4 were ultrasonic treatment for 1, 2, and 5 minutes, accordingly. Table 1 shows the main technological procedures for processing the manufactured samples based on MWCNTs.

Methods
Principle scheme of the THz imaging system [19] for heterogeneous materials with a placed planar positioning mechanism in quasi-optical path of the Mach-Zehnder interferometer is shown in figure 3. The source of THz radiation was a backward wave oscillator (BWO). The object under research was placed in sample holder of the planar positioning mechanism. The sample was moved in a 2D-plane by the point to point method relative to a quasi-optical diaphragmatic beam with a diameter of 2 mm. By BWO with a wavelength of 343 µm and with an average power supply of 2 mW continuous THz wave was generated. An optoacoustic sensor based on a Golay cell recorded the power of the electromagnetic response of the transmitted radiation. Control of the measurement procedure, as well as image visualization in real-time, was performed via the L-Card E154 digital input/output module with using software created in the LabVIEW.
To analyze degree of homogeneity of samples, the evaluation of filtered areas of the obtained THz image by intensity was used. The evaluation method is to measure the mean amplitude of the electromagnetic response. Then the localization of areas of the composite sample with the amplitude of the electromagnetic response differs from the mean value was carried out. Finally, the ratio of localized areas in relation to the area in question was estimated.

Results
To analyze the uniformity of the samples and to compare the evaluation methods the optical microscopy was used. Simultaneously with the illumination from below, the microscope objective was directed at the sample from above. The microscopy result (figures 4 and 5) is shown only for two samples for clarity.   Obtained results of optical microscopy insufficiently determine the degree of uniformity of composite samples treated with ultrasonic. Due to the inapplicability of optical methods for imaging samples with MWCNTs, it became necessary to use penetrating radiation. Figure 6 presents the THz image of the two-dimensional matrix of amplitude radiation at 874 GHz that passed through sample No. 1 based on MWCNTs, which was not ultrasonic processed. And also figures 7, 8 and 9 illustrate THz images of sample No. 2, sample No. 3 and sample No. 4 based on MWCNTs, which were additionally mixed with an ultrasonic dispersant for 1, 2, and 5 minutes, respectively.      Figure 11 shows the ultrasonic treatment time dependence of heterogeneity areas of the composites.

Conclusion
The research showed that the uniformity of MWCNTs distribution in composites depends on the ultrasonic treatment time. Long-term ultrasonic treatment improves material homogeneity. However, as was shown in [20], when the ultrasonic treatment time was increased over 3 minutes, the fact of MWCNTs shortening due to their destruction, as well as a decrease in the dielectric permittivity of the material, was observed. Based on these observations, it was decided to limit the maximum time of ultrasonic treatment to 5 minutes. Thus, this research shows (figure 11) that if ultrasonic treatment time up to 2 minutes, the reduction rate of inhomogeneous areas is 6.65 %/min, and from 2 to 5 minutes it is 1.13 %/min. The possibility of using a BWO-based continuous wave THz imaging system to evaluate homogeneity in composite manufacturing is also shown. Optical microscopy is inferior to THz imaging methods in assessing the homogeneity of composites based on MWCNTs.